The present invention relates to an oxidation method of oxidizing surfaces of workpieces, such as semiconductor wafers, and an oxidation system.
Generally, a semiconductor wafer, such as a silicon substrate, is subjected to various processes including a film forming process, an etching process, an oxidation process, a diffusion process and a modification process when fabricating a semiconductor integrated circuit. For example, the oxidation process among those processes is used for oxidizing a surface of a single-crystal silicon film or a polysilicon film and for oxidizing a metal film. The oxidation process is used mainly for forming gate oxide films and insulating films for capacitors.
Oxidation methods are classified by pressure into atmospheric pressure oxidation methods that are carried out in an atmospheric atmosphere and vacuum oxidation methods that are carried out in a vacuum atmosphere. Oxidation methods are classified by oxidizing gas into wet oxidation methods including a wet oxidation method disclosed in, for example, JP-A No. Hei 3-140453, that use steam generated by burning hydrogen in an oxygen atmosphere in an external combustor, and dry oxidation methods including a dry oxidation method disclosed in, for example, JP-A No. Sho 57-1232 that supply only ozone or oxygen into a processing vessel.
In view of quality and characteristics including dielectric strength, corrosion resistance and reliability, an insulating film formed by a dry oxidation process is superior to that formed by a wet oxidation process. In view of deposition rate and uniformity, generally, an oxide film (insulating film) formed by an atmospheric oxidation process is satisfactory in oxidation rate but the same is not satisfactory in the intrafilm thickness uniformity of an oxide layer formed on the surface of the wafer. On the other hand, an oxide film formed by a vacuum oxidation process is satisfactory in the intrafilm thickness uniformity of the oxide layer but the same is not satisfactory in oxidation rate.
Design rules that have been hitherto applied to designing semiconductor integrated circuits have not been very severe the aforesaid various oxidation methods have been selectively used taking into consideration purposes of oxide films, process conditions and equipment costs. However, line width and film thickness have been progressively decreased and severer design rules have been applied to designing semiconductor integrated circuits in recent years, and design rules requires higher film characteristics and higher intrafilm thickness uniformity of films. The conventional oxidation methods are unable to meet such requirements satisfactorily.
A wet oxidation system disclosed in, for example, JP-A No. Hei 4-18727 supplies H2 gas and O2 gas individually into a lower region in a vertical quartz processing vessel, burns the H2 gas in a combustion space defined in a quartz cap to produce steam, makes the steam flow upward along a row of wafers to accomplish an oxidation process.
Since this prior art oxidation system burns H2 gas in the combustion space, a lower end region in the processing vessel has a high steam concentration, the steam is consumed as the same flows upward and an upper end region in the processing vessel has an excessively low steam concentration. Accordingly, the thickness of an oxide film formed on the surface of the wafer is greatly dependent on the position where the wafer is held for the oxidation process and, in some cases, the intrafilm thickness uniformity of the oxide film is deteriorated.
Another oxidation system disclosed in, for example, JP-A No. Sho 57-1232 arranges a plurality of wafers in a horizontal batch-processing reaction tube, supplies O2 gas or supplies O2 gas and H2 gas simultaneously through one of the opposite ends of the reaction tube into the reaction tube., and forms an oxide film in a vacuum atmosphere.
However, since this prior art oxidation system forms a film in an atmosphere of a relatively high pressure by a hydrogen burning oxidation method, steam is a principal element of reaction, an upper region with respect to the flowing direction of gases in the processing vessel and a lower region in the processing vessel differ excessively from each other in steam concentration and hence it is possible that the intrafilm thickness uniformity of the oxide film is deteriorated.
A third oxidation system disclosed in, for example, U.S. Pat. No. 6,037,273 supplies O2 gas and H2 gas into the processing chamber of a wafer-fed processing vessel provided with a lamp heating device, makes the O2 gas and the H2 gas interact in the vicinity of the surface of a semiconductor wafer placed in the processing chamber to produce steam, and forms an oxide film by oxidizing the surface of the wafer with the steam.
However, this oxidation system supplies O2 gas and H2 gas through gas inlets spaced a short distance in the range of 20 to 30 mm from the wafer into the processing chamber, makes the O2 gas and the H2 gas interact in the vicinity of the surface of the semiconductor wafer to produce steam, and forms the oxide film in an atmosphere of a relatively high process pressure. Thus, it is possible that the intrafilm thickness uniformity of the film is deteriorated.
The present invention has been made to solve the aforesaid problems effectively. Accordingly, it is an object of the present invention to provide an oxidation method and an oxidation system capable of improving the intrafilm thickness uniformity of the oxide film and the interfilm thickness uniformity of oxide films and the characteristics of oxide films, maintaining oxidation rate on a relatively high level.
According to the present invention, an oxidation method of oxidizing surfaces of workpieces heated at a predetermined temperature in a vacuum atmosphere created within a processing vessel comprises the steps of: producing active hydroxyl species and active oxygen species; and oxidizing the surfaces of the workpieces by the active hydroxyl and the active oxygen species.
In the oxidation method according to the present invention, an oxidative gas and a reductive gas are supplied into the processing vessel by separate gas supply systems, respectively, in the step of producing active hydroxyl and active oxygen species.
In the oxidation method according to the present invention, the processing vessel has a predetermined length, the workpieces are arrange at predetermined pitches in a processing region in the processing vessel, an oxidative gas and a reductive gas are supplied into the processing vessel so as to flow from one end of opposite ends of the processing vessel toward the other end of the processing vessel in the step of producing active hydroxyl and active oxygen species.
In the oxidation method according to the present invention, parts of the processing vessel through which the oxidative gas and the reductive gas are supplied into the processing vessel are positioned a predetermined distance apart from the processing region of the workpieces in the processing vessel.
In the oxidation method according to the present invention, the predetermined distance is determined such that the oxidative gas and the reductive gas do not affect adversely temperature distribution in the processing region of the workpieces and the oxidative gas and the reductive gas supplied into the processing vessel can be satisfactorily mixed.
The separation of the parts of the processing vessel through which the oxidative gas and the reductive gas are supplied into the processing vessel from the processing region by the predetermined distance prevents the oxidative gas and the reductive gas from adversely affecting temperature distribution in the processing region in which the workpieces are processed and enables the satisfactory mixing of the oxidative gas and the reductive gas.
In the oxidation method according to the present invention, the predetermined distance is about 100 mm or above.
In the oxidation method according to the present invention, the oxidative gas contains one or some of O2, N2O, NO and NO2, and the reductive gas contains one or some of H2, NH3, CH4 and HCl.
Both the intrafilm thickness uniformity and the characteristics of the oxide film can be improved, maintaining oxidation rate on a relatively high level.
In the oxidation method according to the present invention, the pressure in the vacuum atmosphere is below 133 Pa (1 Torr).
In the oxidation method according to the present invention, the pressure in the vacuum atmosphere is in the range of 6.7 to 67 Pa (0.05 to 0.5 Torr).
In the oxidation method according to the present invention, the predetermined temperature is in the range of 400 to 1100xc2x0 C.
In the oxidation method according to the present invention, an additional oxidative gas and an additional reductive gas are supplied additionally into the processing vessel so as to flow in an opposite direction of the main oxidation gas and the main reductive gas in the step of producing active hydroxyl and active oxygen species.
An oxidation system according to the present invention comprises: a processing vessel for containing workpieces; a support means for supporting workpieces in a processing region in the processing vessel; a heating means for heating workpieces; an evacuation system for evacuating the processing vessel; an oxidative gas supply system for supplying an oxidative gas into the processing vessel; and a reductive gas supply system separate from the oxidative gas supply system, for supplying a reductive gas into the processing vessel to produce active hydroxyl and active oxygen species by the interaction of the oxidative gas and the reductive gas; wherein surfaces of workpieces placed in the processing region are oxidized by the active hydroxyl and the active oxygen species.
In the oxidation system according to the present invention, the oxidative gas supply system and the reductive gas supply system are connected to one end of the processing vessel to make the oxidative gas and the reductive gas flow toward the other end of the processing vessel.
In the oxidation system according to the present invention, the heating means heats both the oxidative gas and the reductive gas.
In the oxidation system according to the present invention, the oxidative gas supply system has an oxidative gas supply nozzle, the reductive gas supply system has a reductive gas supply nozzle, the oxidative gas supply nozzle and the reductive gas supply nozzle have outlets positioned a predetermined distance apart from the processing region of the workpieces.
In the oxidation system according to the present invention, the predetermined distance is determined such that the oxidative gas and the reductive gas do not affect adversely temperature distribution in the processing region of the workpieces and the oxidative gas and the reductive gas can be satisfactorily mixed.
The separation of the outlets of the oxidative gas supply nozzle and the reductive gas supply nozzle from the processing region by the predetermined distance prevents the oxidative gas and the reductive gas from adversely affecting temperature distribution in the processing region in which the workpieces are processed and enables the satisfactory mixing of the oxidative gas and the reductive gas.
In the oxidation system according to the present invention, the predetermined distance is about 100 mm or above.
In the oxidation system according to the present invention, the oxidative gas supply system has an oxidative gas supply nozzle, the reductive gas supply system has a reductive gas supply nozzle, and both the nozzles extend from one of the opposite ends of the processing vessel toward the other end of the processing vessel and have gas outlets positioned at the other end of the processing vessel
In the oxidation system according to the present invention, the oxidative gas contains one or some of O2, N2O, NO and NO2, and the reductive gas contains one or some of H2, NH3, CH4 and HCl.
In the oxidation system according to the present invention, the oxidative gas supply system has a supplementary oxidative gas supply nozzle, the reductive gas supply system has a supplementary reductive gas supply nozzle, and the supplementary oxidative gas supply nozzle and the supplementary reductive gas supply nozzle have gas outlets disposed at one end of the processing vessel.